Data bus systems for vehicles are increasing in importance with respect to vehicles. The cause is the continuously growing number of highly automated, complicated and costly subsystems that communicate with one another, in other words, exchange data. In the area of motor vehicles, for example, these types of subsystems are known as digital engine electronics, electronic hydraulic transmission control and anti-lock bus brake systems. Each of these subsystems is able to transmit data that it obtains on the basis of output signals of connected sensors or switching means and for its work requires data that are available from other subsystems.
Data bus systems of multiple designs are known. However, a problem exists in the control of the access of the subsystems to the data bus, depending upon which access method and which bus control is used. Access methods are, for example, controlled or random. In the case of the controlled access method, the access of the subsystems to the data bus is fixed by a superset control. In the random access method (CSMA=Carrier Sense Multiple Access), a transmission-ready user monitors the bus in order to determine whether at that moment another user is transmitting. If this is not the case, it will start the transmission of messages. Compared to the controlled access method, this access method has the advantage of lesser control expenditures. However, there may be unintentional transmitting by several users when they monitor the data bus at the same time (within the signal run time). So that the transmitter is certain that the message was received correctly, each message must be acknolwedged by the receiver.
In order to avoid a collision of two transmitters, a modification of the above data bus system is known. In this method, known as CSMA/CD (CD =Collision Detection), the overlay of two transmitters is recognized by corresponding detectors in each transmitter. Access conflicts are eliminated by rollback attempt time intervals that differ in each transmitter. Random access methods therefore require control expenditures in order to function reliably. As compared to the controlled access methods, the random access methods have the advantage that they work independent of the number of users, whereas, in the case of a controlled access method, the number of users and their access sequence must be fixed.
In the case of a data bus system for vehicles, wherein various users communicate with one another via a common data bus, the objective arises of designing a system independently of the number of users reliably and with low expenditures with respect to control and switching technology.
In the present invention, the access of the individual users to the data bus for the transmitting and the receiving of the data is possible at any time and is ensured by the tag of the data supplied to the data bus. The transmitting of the data by the station assigned to each user takes place in an uncontrolled way to the extent that the receiver of these data is not necessarily known to the transmitting station. Concerning the transmitter, there is only a control to the extent that each user transmits its data sequentially, and the stations themselves are switched sequentially to the data bus. Similarly to the mentioned SCMA/CD method, the data bus has the advantage of modularity. The failure of a user or the absence of a user plays as little a role as the adding or the exchange of a user.
The control expenditures of this type of open system are low. They consist essentially only of controlling the sequence of the individual users for the transmitting of their data to the data bus. Advantageously, this may take place centrally. In contrast to a data bus system that is known under the name "token passing" and where the authorization to transmit is passed along from user to user, it can result in the avoidance of safety measures in the case of a user's breakdown. A safety measure of this type is known, for example, from DE 3,222,570 Al in which each user not only monitors the transmitting operation of the transmitter that precedes it in sequence, but also the transmitting operation of the transmitter preceding that transmitter.
A further improvement is achieved in preferred embodiments if the central control takes place by the station with the highest transmitting priority. In contrast to a control by a higher-ranking control unit, this results in a reduction of the expenditure with respect to components. In addition, this can ensure that, in the case of a breakdown of the control unit, the transmitting operation is controlled by the station with the then highest control priority and, as a result, a reliable transmitting operation can be maintained.
Further improvements of the data bus system are achieved if the stations send their data messages in variable lengths determined by the number of data messages to be transmitted and their length. In the case of a data bus system of that type, the data bus is occupied only to such an extent that the exchange of data is actually required. This also provides the possibility of selecting the frequency of transmission differently for each station as a function of the respective data each station transmits. If a station has data that change with different frequency, the station can transmit only the data that change frequently, whereas the data that rarely change are contained only in very few of the data messages. This frequency with which the stations transmit their data may, for example, be determined by the frequency of change of the data themselves.
In the case of a data bus system of this type, the data as a rule are filed or renewed at non-reproducible points in time. It may be advantageous in these cases for the stations to exchange these data with the pertinent user irrespectively of the frequency with which the stations are connected to the data bus for the transmitting of data or with which the data are transmitted on the data bus. It must only be ensured by corresponding replacement data or replacement data that are present in the stations that these replacement data, if the actual data fail, are used instead of the actual data.
Further objects, features, and advantages of the present invention will become more apparent from the following description when with the accompanying drawings which show, for purposes of illustration only, an embodiment in accordance with the present invention.